This invention relates to mechanical wire mesh isolators and similar bushings suitable for high temperature use and to methods and apparatus for making the same.
Devices made from knitted wire mesh are commonly used as seals, bushings, seats, and supports in and around exhaust systems for internal combustion engines. In these types of devices, a wire is knitted into a mesh, and the mesh is compacted in a die into a particular shape. Sometimes the compaction is partial, and the partially-compacted structure can be infiltrated with a high temperature resistant material, such as described in U.S. Pat. No. 5,385,873 (the disclosure of which is incorporated herein by reference). At other times the compaction results in a denser article that can be used as a bushing at the end of a catalytic converter, such as described in U.S. Pat. Nos. 4,683,010 and 6,286,840 (the disclosures of which are incorporated herein by reference). Still other devices are even more densely compacted and can be used as a filter in an air bag assembly, as described in U.S. Pat. No. 6,277,166 (the disclosure of which is incorporated herein by reference).
The combustion that takes place in an internal combustion engine of an automotive vehicle produces substantial amounts of heated noxious gas and significant amounts of noise. As a result, all automotive vehicles include an exhaust system that transports the exhaust gas from the engine to a location on the vehicle where the heat exhaust gas can be emitted safely. The exhaust system includes a converter to catalyze compounds in the exhaust gas into less noxious gases. Other components of the exhaust system function to attenuate the noise associated with the flowing and rapidly expanding gases produced by the combustion processes in the engine.
The typical exhaust system extends from the engine compartment near the front of the vehicle to a location at or near the rear of the vehicle where the exhaust gases may be emitted safely. The exhaust system typically includes a plurality of pipes, a catalytic converter and at least one muffler. These various components of the vehicle must compete for space on the underside of the vehicle with other necessary components of the vehicle. There is an even higher premium on space in the engine compartment, where heat shields are used around exhaust manifolds and close coupled converters (that is, catalytic converters that are mounted very near the exhaust manifold). The entire exhaust system becomes very hot after a short period of operation due to the high temperatures generated during the combustion processes that produce the exhaust gas and the catalytic reaction in the converter. The realities of designing an exhaust system to fit into the limited space on the underside of a vehicle typically positions certain components of the exhaust system close to passenger compartments, luggage compartments or other heat sensitive components or sections on the vehicle. As a result, most exhaust systems must include at least one heat shield, including a heat shield near the muffler.
The typical heat shield for a muffler is a thin sheet of metal that is stamped or otherwise formed to conform generally to the shape of the muffler. The heat shield may be formed with legs or other structures that provide small areas for attaching the heat shield to the muffler. A major portion of the typical heat shield is spaced from the outer shell of the muffler to provide an air gap that will insulate sensitive areas of the vehicle from the heated muffler. The heat shield typically is secured to the muffler by welding. However, other attachment means, such as straps, rivets, bolts or folded seams have been employed in the prior art.
Heat shields can be designed to perform their primary heat shielding function adequately. Parts of the heat shield necessarily must be spaced from the muffler to perform the heat shielding function. As a result, the heat shield is substantially cooler than adjacent areas of the muffler. The temperature differential between the heat shield and the muffler leads to differential thermal expansion. Therefore, the use of weldments or other such attachments between the heat shield and the muffler are subject to substantial and repeated forces as the muffler goes through its heating and cooling cycles. Additionally, the entire exhaust system is subject to significant vibration during use. Consequently, welded attachments between the heat shield and the muffler are subject to failure. A failed connection will cause the heat shield to vibrate against the exterior of the muffler and/or against other nearby parts of the vehicle. Such vibrations can create very objectionable noise. A folded connection between the heat shield and the muffler can be designed to accommodate some motion during differential thermal expansion without adversely affecting the long term connection between the muffler and the heat shield. However, folds or other such mechanical connections also are subject to vibration during use and hence can generate objectionable noise.
To overcome some of these problems, a two part wire mesh isolator or bushing assembly is used, as shown in FIGS. 1 and 2. These usually consist of a hex head cap screw (A) and a solid headed, metal, barbed grommet (B). In combination, the screw and the grommet hold together the two parts, a wire mesh collar (C) and a wire mesh grommet (D) of the isolator. In use, the two wire mesh parts, the collar and mesh grommet, of the isolator must be located and held in place adjacent a bore in the heat shield (H). The solid grommet is located through the collar into the mesh grommet, and inserting the screw into the support fitment (F) (attached to a mock-up of an exhaust manifold in the figure) which compresses the mesh into the barbs on the grommet to lock the two wire mesh parts together by friction. However, the vibration and thermal expansion during use of the vehicle can cause the mesh to unwind. There is also the problem of having to locate the two mesh parts in place during assembly. Yet another problem occurs: when assembled heat shields are shipped to the end user (manufacturer), the barbed solid grommet works it way out during transportation and handling, so the shield reaches the production line without the grommet and the mesh components.